Discovery and Evolutionary Analysis of a Novel Genus of Endogenous Pararetroviruses with Long Terminal Repeats

Despite being widespread in plants, endogenous pararetroviruses (EPRVs) are still poorly understood in barley and many other cereal crops. In this study, the barley reference genome was examined and a new EPRV was identified and named Hvu-EPRV. In contrast to all EPRVs identified thus far, Hvu-EPRV contains long terminal repeats (LTRs) which are similar to LTR retrotransposons. Homologous sequences of Hvu-EPRV were found in a wide range of plants, however, only those in 17 grasses belonging to the six tribes contain LTRs. The insertion times of nested LTR retrotransposons indicated that Hvu-EPRVs inserted into barley more than 2.37 million years ago, but the invasion and endogenization of Hvu-EPRV related elements in the grass family may be ancient, and horizontal transfers may have occurred between grasses. Phylogenetic analysis revealed that Hvu-EPRV and its homologs in grasses were grouped apart from all 13 reported genera of exogenous and EPRVs, thus the EPRVs in grasses represent a novel genus of the Caulimoviridae family named Idahovirus. Genome-wide comparisons of Hvu-EPRVs were conducted between the reference genome and other 84 genomes of cultivated and wild barley, three independent integration events were observed and suggested that the integrations likely occurred after the divergence between barley and its wild progenitor. This is the first time to identify EPRVs with LTRs and to detect their recent integrations, and this research provides new insights into the evolution of plant EPRVs and their invasion history in the grass family.